The present invention is directed to an explosive powder charge operated setting tool containing a piston guide forming a guide bore with a cartridge carrier at one end for receiving an explosive powder charge capable of propelling a piston by means of propellant gases from a rear starting position to a front end position. The cartridge carrier forms a rear stop for the piston after it is returned from the front end position. A channel located between the piston guide and a laterally enclosing housing part communicates through openings with the guide bore in the rear starting position and in the front end position.
In explosive powder charge operated setting tools of this type a piston is driven from the rear starting position into the front end position by propellant gases generated when the explosive powder charge is ignited. The piston drives bolts, nails and similar fastening elements directly into hard receiving materials, such as concrete, metal and the like.
To return the piston, after it has completed the driving operation, back into the starting position for the next driving operation, it is necessary to move the piston from its front end position to its rear starting position. In known tools this operation is effected in many ways.
For instance, it has been known to move the piston by means of a separate tappet or ram into its rear starting position. Such an operation has the disadvantage that it requires considerable time and involves the use of a separate tappet which can be easily lost.
Further, a mechanical return arrangement for the tool has been widely used and involves pulling the piston guide forward over the piston in its end position and subsequently moving the piston guide along with the piston rearwardly, whereby the piston ends up in the rear starting position.
This piston return procedure also requires a relatively long time period due to the special manipulating step, and is especially disadvantageous in multiple or series fastening element setting operations, such as performed by setting tools of the above type.
To avoid such special manipulating steps or handling operations, it has been known to effect the piston return by using propellant gases such as disclosed in EP 0 223 740. In this type of piston return, an opening is exposed in the region of the starting position of the piston, after the piston has begun to move following ignition of the powder charge, so that some of the propellant gases flow back into the guide bore in the piston guide through a channel and another opening in the region of the end position of the piston. These propellant gases, which have entered into the front end of the guide bore in the piston guide, are compressed by the piston and serve to drive the piston as they expand, after the completion of the driving step, returning the piston from the front end position into its rear starting position.
This known piston return operation has the advantage that it takes place completely automatically without any additional manual operation. The elimination of the mechanical operations, however, involves certain problems, whereby depending upon the quantity of the propellant gases the piston is driven with more or less force against the rear stop formed by the cartridge carrier. If the compressed propellant gases provide a strong returning force, there can be the disadvantage that a rebound effect is generated when the piston impacts the cartridge carrier and, as a result, the piston again moves over a certain distance in the driving or setting direction due to the rebound effect and does not assume its original rear starting position. Such an effect results in the development of an open or dead space with respect to the rear starting position, so that the desired output of the setting tool is not attained or the opening located in the region of the starting position remains open and a large share of the propellant gases escapes and is not available for driving the piston.